i'd say grow him out to 12" and then sell him if he's got nice colours. i was fishing in a pond in the mountains above oak hurst and caught a goldfish that was 14" from the tip of the nose to the base of the tail, w/ tailfin it was nearly 17"(i'm not exaggerating, i had a tape measure) i also caught about 15 others between 8" and 12" for a goldfish that large anywhere from $10 to $20 would probably be a good price if a LFS was selling it, when a private individual is selling people tend to not want to pay retail prices.